In the well-known process of electrostatographic printing, also known as "xerography," a charge retentive surface, typically known as a photoreceptor, is electrostatically charged, and then exposed to a light pattern of an original image to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on the photoreceptor form an electrostatic charge pattern, known as a latent image, conforming to the original image. The latent image is developed by contacting it with a finally divided electrostatically attractable powder known as "toner." Toner is held on the image areas by the electrostatic charge on the photoreceptor surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate, such as paper, and the image affixed thereto to form a permanent record of the image to be reproduced.
The step in the electrophotographic process in which the toner is applied to the latent image is known as "development." In any development system, a quantity of toner is brought generally into contact with the latent image, so that the toner particles will adhere or not adhere to various areas on the surface in conformity with the latent image. Many techniques for carrying out this development are known in the art. A number of such techniques require that the toner particles be evenly mixed with a quantity of "carrier." Generally speaking, toner plus carrier equals "developer." Typically, toner particles are extremely fine, and responsive to electric fields; carrier particles are relatively large and respond to magnetic fields. In a "magnetic brush" development system, the developer is exposed to relatively strong magnetic fields, causing the carrier particles to form brush-like strands, much in the manner of iron filings when exposed to a magnetic field. The toner particles, in turn, are triboelectrically adhered to the carrier particles in the strands. What is thus formed is a brush of magnetic particles with toner particles adhering to the strands of the brush. This brush can be brought in contact with the latent image, and under certain conditions the toner particles will separate from the carrier particles and adhere as necessary to the photoreceptor.
An important process parameter for any development system is the ratio of toner particles to carrier within the developer. It is also expectable that, in the course of use of the printer, the toner to carrier ratio (T/C) will change significantly as toner particles are transferred from the developer supply to the photoreceptor and ultimately to print sheets. There have thus been numerous systems devised in the prior art for determining and controlling this T/C in an operating machine. Because carrier particles are generally heavy and magnetic, while toner particles are generally light and non-magnetic, many of these systems involve detecting the behavior of magnetic flux through the developer; placing a quantity of developer between capacitor plates and examining the electrical behavior thereof; or electrically drawing a quantity of toner from the developer and inferring a T/C therefrom. However, very often such systems have proven to be either inaccurate, imprecise, or too expensive for use in inexpensive printers and copiers.
The present invention is directed to a highly precise system for monitoring and controlling the T/C in a developer supply.